A LONG-LIVED ENRICHED MANTLE SOURCE FOR 2 PROTEROZOIC CARBONATITE COMPLEXES FROM TAMIL-NADU, SOUTHERN INDIA

We report new neodymium and strontium isotopic data for two Proterozoi c carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 G a (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neod ymium isotopic compositions, together with oxygen isotope data, sugges t the involvement of a single long-lived enriched mantle source in the ir origin. The isotopic evolution of this source indicates that it for med approximately contemporaneously with the accretion and metamorphis m of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoi c until at least 770 Ma ago. The older of the two carbonatites was int ruded into young crust that was not older than about 150 Ma at the tim e of emplacement. The isotopic data contrast with those from carbonati tes of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochem ically unique lithospheric source for carbonatites. Copyright (C) 1998 Elsevier Science Ltd.